A background art of the present invention is explained with reference to a shovel shown in FIG. 3 as an example. The shovel includes a lower traveling body 1 of a crawler type, an upper slewing body 2 mounted so as to be capable of slewing around an axis X perpendicular to the ground, and a work attachment 3 attached to the upper slewing body 2. The work attachment 3 includes a boom 4 capable of being raised and lowered, an arm 5 attached to the distal end of the boom 4, a bucket 6 attached to the distal end of the arm 5, and a plurality of hydraulic cylinders for actuating the boom 4, the arm 5, and the bucket 6, respectively, namely, a boom cylinder 6, an arm cylinder 7, and a bucket cylinder 8. The shovel further includes a traveling motor, which is a hydraulic motor for causing the lower traveling body 1 to travel, and a slewing motor, which is a hydraulic motor for slewing the upper slewing body 2.
In the hydraulic shovel, during slewing deceleration, energy due to the inertia of the upper slewing body 2 is applied to the slewing motor. Besides, a load in a boom lowering direction due to the gravity acting on the attachment 3 or the like constantly acts on the boom cylinder 7, which constantly produce pressure in a fluid chamber of the boom cylinder 7 into which hydraulic fluid for extending the boom cylinder 7 is introduced. The fluid discharged from the fluid chamber has certain energy.
As means for effective utilization of such energy of a hydraulic actuator, there are known respective apparatuses described in Patent Literature 1 and 2. Each of techniques involves a regenerative motor connected to an engine. The regenerative motor is driven to rotate with fluid discharged from the hydraulic actuator to assist the engine. Alternatively, there is also known a hybrid shovel including a regenerative motor, a generator motor and an electric storage apparatus, wherein the regenerative motor drives the generator motor to thereby assist the engine and generated electric power is stored in the electric storage apparatus.
FIG. 4 shows a publicly-known technique described in Patent Literature 1. For simplification of explanation, FIG. 4 shows only constituent elements concerning slewing.
FIG. 4 shows an apparatus, which includes an engine 10, a hydraulic pump 11 functioning as a hydraulic pressure source driven by the engine 10, a slewing motor 12 which is rotated by pressure fluid from the hydraulic pump 11 to slew the upper slewing body 2, and a control valve 13 provided between the hydraulic pump 11 a tank T and the slewing motor 12. The control valve 13 is a hydraulically pilot controlled selector valve including a pair of pilot ports for receiving supply of a pilot pressure from a not-shown remote control valve, the selector valve being selectively operated by the pilot pressure. The control valve 13 changes supply-and-discharge state of hydraulic fluid to and from the slewing motor 12 to thereby enable control of an operation state of the slewing motor 12, specifically, control of rotation/stop, a rotating direction, and rotating speed of the slewing motor 12, to be performed.
Specifically, the control valve 13 has a neutral position 13a, a leftward-slewing position 13b, and a rightward-slewing position 13c. When no pilot pressure is supplied from the remote control valve to either of the pilot ports, the control valve 13 is retained in the neutral position 13a. When a pilot pressure is supplied from the remote control valve to any one of the pilot ports, the control valve 13 is shifted to a selected position of the leftward-slewing position 13b and the rightward-slewing position 13c, the selected position corresponding to the pilot port to which the pilot pressure is supplied.
In the neutral position 13a, the control valve 13 blocks a left-side slewing conduit 14 and a right-side slewing conduit 15, which connect the control valve 13 to left and right ports of the slewing motor 12, respectively, from the hydraulic pump 11, thereby hindering the slewing motor 12 from rotation. When shifted to the leftward-slewing position 13b by an operation applied to the remote control valve to a left slewing side, the control valve 13 allows the hydraulic fluid to be supplied from the hydraulic pump 11 to the leftward-slewing conduit 14, thereby rotating the slewing motor 12 leftward and slewing the upper slewing body 2 leftward. Conversely, when shifted to the rightward-slewing position 13c by operation applied to the remote control valve a right slewing side, the control valve 13 allows the hydraulic fluid from the hydraulic pump 11 to be supplied to the rightward-slewing conduit 15, thereby rotating the slewing motor 12 rightward to slew the upper slewing body 2 rightward.
The apparatus further includes a brake circuit 21. The brake circuit 21 includes left and right relief valves 16 and 17 provided as respective hydraulic brake valves and opposed to each other, left and right check valves 18 and 19 for anti-cavitation (for fluid suction) provided in parallel to the left and right relief valves 16 and 17 and opposed to each other, and a passage 20 interconnecting respective outlet ports of the left and right relief valves 16 and 17 and respective inlet ports of the left and right check valves 18 and 19. The hydraulic brake circuit 21 performs anti-cavitation action of returning fluid on a meter-out side to a meter-in side of the slewing motor 12 during slewing deceleration to prevent cavitation from occurrence and performs hydraulic brake action by the left and right relief valves 16 and 17.
Although not described in Patent Literature 1 and 2, the passage 20 of the hydraulic brake circuit 21 is usually connected to the tank T through a makeup line 22 for fluid pump-up. The makeup line 22A is provided with a back pressure valve (a boost check valve) 23, which produces a fixed back pressure, and an fluid cooler 24.
In the apparatus shown in FIG. 4, when returned to the neutral position 13a from, for example, the leftward-slewing position 13b, the control valve 13 separates the slewing motor 12 and both the slewing conduits 14 and 15 from the hydraulic pump 11 and the tank T to stop the supply of the hydraulic fluid to the slewing motor 12 and return of the hydraulic fluid from the slewing motor 12 to the tank T. However, the upper slewing body 2 continues the leftward slewing due to the inertia thereof, involving the slewing motor 12 to continue the rotation to produce pressure in the rightward-slewing conduit 15, which is a meter-out side conduit. When the pressure reaches a fixed value, the right relief valve 17 is opened to allow the hydraulic fluid in the rightward-slewing conduit 15 to flow into the slewing motor 12 passing through the right relief valve 17, the passage 20, the left check valve 18, and the leftward-slewing conduit 14, which is a meter-in conduit, in order.
Furthermore, when the pressure in the leftward-slewing conduit 14 is increased, the leftward-slewing conduit 14 sucks up the hydraulic fluid in the tank T through the makeup line 22 and the check valve 18 to thereby prevent cavitation. Thus, anti-cavitation act is automatically performed. The suction of the hydraulic fluid further applies a brake force to the slewing motor 12 rotated by the inertia of the upper slewing body 2 and thereby stops the slewing motor 12 gently. The action explained above is performed in the same manner during return of the control valve 13 from the rightward-slewing position 13c to the neutral position 13a. FIG. 4 indicates a flow of the fluid during the leftward slewing by white arrows and a black arrow, and indicates a flow of the hydraulic fluid for anti-cavitation by the black arrow.
The apparatus further includes a regenerative motor 25, which is a hydraulic motor for regeneration, a regeneration selector valve 26, a left regeneration line 27 and a right regeneration line 28. The regenerative motor 25 is coupled to the engine 10 and includes an inlet port connected to the regeneration selector valve 26 and an outlet port connected to the tank T. The regeneration selector valve 26 includes a pair of inlet ports connected to the left and rightward-slewing conduits 14 and 15 via the left and right regeneration lines 27 and 28, respectively, and the an outlet port connected to the regenerative motor 25.
The regeneration selector valve 26 has a neutral position 26a for blocking the regenerative motor 25 from the left and right regeneration lines 27 and 28, a left regeneration position 26b for connecting the regenerative motor 25 to the left regeneration line 27, and a right regeneration position 26c for connecting the regenerative motor 25 to the right regeneration line 28. These positions are selected according to a command input from a not-shown controller on the basis of operation of the remote control valve. The regeneration selector valve 26 is shifted to the left regeneration position 26b, for example, during leftward slewing deceleration, thereby allowing the hydraulic fluid discharged from the slewing motor 12 to flow into the regenerative motor 25 through the rightward-slewing conduit 15, which is the meter-out side conduit, the right regeneration line 27, and the regeneration selector valve 26 and to thereby rotate the regenerative motor 25. The driving of the regenerative motor 25 makes it possible to regenerate energy of the hydraulic fluid as rotational energy (in this case, as an engine assist force) to thereby enable the energy efficiency of a system to be improved.
However, in the apparatus wherein the hydraulic fluid discharged from the regenerative motor 25, namely, regeneration discharge fluid, is always directly returned to the tank T, the hydraulic fluid discharged from the slewing motor 12 during slewing deceleration returns to the tank T through the regenerative motor 25 without being supplied to the meter-in side, thereby permitting cavitation to be caused. This could be prevented by connecting an outlet side of the regenerative motor 25 to the makeup line 22 and returning the regeneration discharge fluid to the tank T through the back pressure valve 23 to produce back pressure; however, thus applying the back pressure to the regenerative motor 25 reduces an effective differential pressure and rotational speed of the regenerative motor 25 to deteriorate regeneration efficiency. Besides, while the hydraulic actuators connected to the regenerative motor 25 include an actuator with no risk of cavitation, the back pressure is uselessly applied also during actuation of the actuator with no risk to deteriorate the regeneration efficiency.
Patent Literature 2 discloses another cavitation prevention means including providing an accumulator as a hydraulic source for anti-cavitation, rotating the regenerative motor 25 with regenerative fluid extracted from the meter-out side of the slewing motor 12 during slewing deceleration, and supplying fluid in the accumulator to the meter-in side as anti-cavitation fluid. However, the technique requires large additional facilities, namely, a dedicated accumulator and an anti-cavitation circuit, thus involving an increase in facility costs and complication of a circuit.